Gem-quality white diamonds greater than 20 carats have comprised around 0.00007% of Argyle's production since 1983.

"Argyle Octavia is unique in its size, shape and beautiful provenance and will take its place in Argyle's history as one of a few special large white diamonds ever to be produced from the mine," Argyle Diamonds general manager Andrew Wilson said.

The rough diamond will be sold by tender in Antwerp later this year.

"We are delighted with this discovery which is a testament to the extraordinary Argyle orebody that continues to deliver these miracles of nature, even as it nears the end of mine life," Rio Copper and Diamonds chief executive Arnaud Soirat said.

Rio has previously said the mine would close next year, but CEO J-S Jacques hinted last month that the end could come sooner than that.

Jacques said he held out hope that a new discovery could keep Argyle going longer.

"I'll continue to put our friends from exploration under massive pressure," he said.

"If they could find me a nice open pit shallow in Australia … shallow, world-class, with lots of pink diamonds, I will be absolutely delighted."